In staged combustion gas turbine engines, the arrangement of the injectors commonly includes a set of pilot injectors, which are on at all times when the engine is running, and one or more sets of main injectors, which are fired in stages as thrust demand increases. Ideally, the system controlling the supply of fuel to the injectors should have the ability to maintain the rate of fuel flow to each set of injectors, even when the flow to other sets is changing. Problems can arise with control systems as a reduction in flow can occur in the fuel supply to a first set of main injectors when a further set of main injectors is switched on.
In staged combustion gas turbine engines, it is also a requirement to be able to purge with air the part of the fuel supply path to a set of injectors exposed to high ambient temperatures when the injectors are switched off, otherwise the paths are liable to block as a result of solid carbon particles forming in the residual fuel. Furthermore, once a path has been purged it is desirable to minimise the delay that occurs in refilling the path when the injectors are next switched on.
An example of a known staged combustion gas turbine engine injector system is shown in FIG. 1. The staged combustion gas turbine engine injector system comprises a fuel metering unit 10, supplying a regulated flow of fuel under high pressure directly to a group of first stage fuel injectors 12 (only one shown), and to a group of second stage fuel injectors 14 (only one shown) through a staging valve 16.
The valve 16 comprises a fuel supply passage or inlet 18 supplied with fuel from the fuel metering unit 10, a fuel outlet 20 to supply fuel to the second stage fuel injector 14, and a valve member in the form of a piston 22 having a piston seal 22a positioned toward one end, the piston 22 being slidingly mounted in a valve chamber 23. The valve 16 further comprises a second passage 24 positioned on the opposite side of the piston 22 to the fuel supply passage 18. The second passage 24 is supplied with fuel for piston control purposes from a high-pressure fuel line 26 via a two-position ball valve 28 controlled by a solenoid 32. The valve 28 also controls the return flow of fuel from the valve chamber 23 via the second passage 24 to a low pressure fuel return path 30 leading to a low pressure fuel reservoir (not shown).
In the position illustrated in FIG. 1, the solenoid 32 is energised so that an armature 33 thereof applies a force to the ball of the ball valve 28 to urge the ball into a position in which the high pressure fuel line 26 is closed and the chamber 23 is in communication with the low pressure fuel return path 30. As a result, the high pressure fuel acting on the piston 22 via the fuel supply passage 18 has caused the piston 22 to move into the position illustrated in FIG. 1 so that high pressure fuel is supplied from the fuel metering unit 10 to the set of second stage injectors 14 via the fuel outlet 20.
When the solenoid 32 is de-energised, the force acting on the ball of the ball valve 28 by the armature 33 is released with the result that the high pressure of fuel in the line 26 moves the ball into a position in which it closes the low pressure return path 30. Since the fuel from the high-pressure fuel source 26 is at a higher pressure than that supplied from the fuel metering unit 10, the pressure variation causes the piston 22 to be positioned in the valve chamber 23 so as to prevent passage of fuel from the fuel metering unit 10 to the second stage fuel injectors 14.
The fuel metering unit 10 provides a continuous flow of fuel to the first stage injectors 12. The second stage injectors 14 are brought into operation when a greater flow of fuel is required than can be supplied by the first stage injectors 12. The flow capacity of the valve 16 is very large relative to the flow capacity of the injectors 14 so that the flow split between the injectors 12, 14 is determined by the restrictions to flow of fuel through the outlet orifices of the injectors. When the second stage injectors 14 are being primed, air rather than fuel passes through the outlet orifices. Consequently, there is no significant pressure drop across the outlet orifices and flow into these injectors is relatively unrestricted during priming. Accordingly there is a temporary reduction in flow of fuel to the first stage fuel injectors due to preferential flow to the second stage injectors whilst they are being primed. This can result in a potentially dangerous temporary loss of power or even flame extinction. This reduction in flow is exacerbated by the loss of metered fuel to the low pressure reservoir as the piston 22 is displaced to the open position.
It is an object of the invention to provide a fuel injection system for use in a combustion engine in which the aforementioned problem is alleviated.